Gaudi Build Update, master branch (2020.04.23.)

This MR is in preparation for gaudi/Gaudi!986 (merged) to become part of the master branch of Gaudi.

I have to admit, I had to do a few more things than I expected...

• I had to update a good number of "find modules" to make it easier to use their outputs as inputs to the Gaudi build configuration.
• I had to teach atlas_build_run.sh how to pass "quoted arguments" through itself. Previously it would shave off quote marks from its arguments, making certain commands fail.
• The configuration of Gaudi needed a bit more effort than I thought... But I think I now have a reasonable setup.

As an interesting "aside": I was having a lot of problems with making the Gaudi build use genreflex successfully. Until I realised that Gaudi was capturing the environment in which the CMake configuration was running. (And not what we do, where we extend that configuration based on what externals we need for the build.) But I was very happy when I realised that I can set things up correctly by specifying what CMAKE_COMMAND the Gaudi configuration should use. (It is this update that highlighted the previous shortcoming of atlas_build_run.sh.)

For now I had to put a "patch" into the build configuration, that would allow Gaudi to build against the version of CLHEP that we use for Athena at the moment. @clemenci, @leggett, do you know of any good reason why the required minimum version of CLHEP couldn't be lowered like this in Gaudi? The build itself at least succeeds just fine with this patch.

Also pinging @wlampl, @emoyse, @fwinkl.